As design rules in CMOS technology have scaled into the deep submicron regime, the performance of CMOS transistors has failed to keep pace. This is due to lower power supply voltages, to higher channel doping (increased series resistance) to reduce short channel effects, and due to reduced scaling of the transistor channel length.
Stress is now commonly used to improve transistor performance. Compressive stress when applied parallel to the current flow increases hole mobility for PMOS transistors thus improving PMOS transistor performance. Tensile stress when applied parallel to the current flow increases electron mobility in NMOS transistors improving performance. Since opposite type stress is required to enhance performance of NMOS and PMOS transistors, dual stress liner (DSL) technology has been developed to put a film with tensile stress over NMOS transistors and with compressive stress over PMOS transistors. In a DSL flow, a tensile silicon nitride film is deposited, patterned, and etched to leave the tensile liner over the NMOS transistors to enhance electron mobility in the NMOS transistor channels. A compressive silicon nitride film is then deposited, patterned, and etched to leave the compressive liner over the PMOS transistors to enhance hole mobility in the PMOS transistor channels.
Compressive stress when applied perpendicular to the current flow degrades both hole and electron mobility and therefore degrades performance of both NMOS and PMOS transistors. Tensile stress when applied perpendicular to the current flow improves both hole and electron mobility and therefore enhances performance of both NMOS and PMOS transistors. The position of the DSL boundary with respect to the transistor channel may be optimized to minimize the negative impact upon NMOS and PMOS transistor performance. Often the position of the DSL boundary is constrained by design rules and layout.
For example, when an NMOS and PMOS transistor are vertically adjacent as shown in FIG. 1, to minimize chip area, minimum design rules for NMOS gate overhang of active 34, PMOS gate overhang of active 36, and gate tip-to-tip space 42 are used. This limits the distance the DSL boundary 32 may be moved from NMOS and PMOS transistors. Typically the DSL boundary is placed midway between the NMOS active 20 and PMOS active 22. Sometimes as is disclosed in U.S. Provisional Application 61/409,583 (Texas Instruments docket number TI-68750, filed Nov. 3, 2010) incorporated herein by reference, it may be offset to improve transistor performance.